1. Field of the Invention
The present invention relates to the use of copolymers which are obtainable by copolymerization of N-vinylcarboxamides with other monoethylenically unsaturated compounds and partial or complete elimination of formyl groups from the N-vinylcarboxamides polymerized in the copolymer, with formation of amino or ammonium groups, as additives to paper stock for increasing the drainage rate and retention in papermaking and the dry and wet strength of the paper.
2. Description of the Background
U.S. Pat. No. 2,721,140 discloses the use of polyvinylamine hydrochloride as an assistant in papermaking, in particular papers having high wet strength being obtained. However, the paper treated in this manner has a relatively high tendency to yellow.
U.S. Pat. No. 4,421,602 discloses partially hydrolyzed, water-soluble homopolymers of N-vinylformamide which contain N-vinylformamide units and vinylamine units. These polymers are used as agents for increasing the flocculation, retention and drainage rate in papermaking.
EP-A-0 438 744 discloses copolymers which contain, as polymerized units, from 99 to 50% by weight of at least one monoethylenically unsaturated carboxylic acid of 3 to 8 carbon atoms or salts thereof and from 1 to 50% by weight of N-vinylcarboxamides, preferably N-vinylformamide, and which have been subjected to hydrolysis in which the formyl groups of the N-vinylcarboxamides are eliminated with formation of amino or ammonium groups. These copolymers are used as water treatment agents for reducing lime-scale and water hardness deposits in water-conveying systems and in the concentration of sugar juice.
EP-A-0 216 387 discloses water-soluble copolymers which contain vinylamine units and are obtainable by copolymerization of N-vinylformamide with monoethylenically unsaturated monomers selected from the group consisting of vinyl acetate, vinyl propionate, C.sub.1 - to C.sub.4 -alkyl vinyl ethers, the esters, nitriles and amides of acrylic acid and methacrylic acid and N-vinylpyrrolidone and elimination of the formyl groups from the copolymers. These copolymers and, preferably, copolymers of N-vinylformamide and vinyl acetate, from 30 to 100 mol % of whose monomer units are hydrolyzed in each case, are used as agents for increasing the dry and wet strength of paper.
EP-B-0 251 182 discloses copolymers which contain vinylamine groups in salt form, vinylformamide groups and acrylonitrile groups and, if required, acrylamide and acrylic acid groups. The amount of polymerized acrylic acid or methacrylic acid is not more than 6.4 mol %. These copolymers are used in papermaking and act as drainage and retention agents and as agents for increasing the dry strength of paper.
It is an object of the present invention to provide a further paper aid which, when added to the paper stock, leads to an increase in the mechanical strengths of the papers thus produced and at the same time increases the drainage rate and retention.